If you want to show a tremolo on only the second note in a tie chain, go to Engrave mode, select the notehead on which you want to show the tremolo strokes, open the Properties panel, and find the ‘Single stem tremolo’ property: enable this and choose the number of strokes you want to see there.
If you really want to enter the tremolo in Write Mode, then do it the other way around from Daniel’s suggestion—enter it in Write Mode, THEN go into Engrave Mode, select the note head (JUST the note head—that’s important and messed me up for a while) where you don’t want the tremolo, and deselect the Single Stem Tremolo property in the Property panel.
I do it this way to avoid the constant back and forth you referred to in your post. Rather than do it on a one-by-one basis, I just put all the tremolos in, then, in Engrave Mode, Cmd-click all the notes where I don’t want tremolos. and change them with one click in the Property panel.